A liquid crystal display apparatus can be decreased in weight and thickness and actuated at a low voltage, and it is therefore actively introduced to office automation machines and equipment such as a personal computer, a word processor, etc. And, a liquid crystal display apparatus having the above advantages is inevitably designed toward a larger screen, an increase in the number of picture element pixels and higher preciseness, and there is desired a high-quality liquid crystal display apparatus free of display defects.
A liquid crystal display device has a sandwich structure in which a liquid crystal is sandwiched between mutually opposing two transparent electrodes, and the transparent. electrodes constitute one important element for obtaining a high-quality liquid crystal display device. The transparent electrode is fabricated, for example, by patterning a transparent electrically conductive layer formed on a transparent glass substrate, to a predetermined form by photolithography. In recent years, for further decreasing the weight of a display apparatus, it is under way to make attempts to substitute polymer films for the transparent glass substrate.
At present, an ITO electrode is the mainstream as a transparent electrode. An ITO layer which constitutes the base of the ITO electrode is formed by a sputtering method using ITO as a sputtering target. It is because an ITO layer not only has high transparency and low resistance but also has excellent etching properties (etching rate) and excellent adhesion to a substrate that a large number of ITO electrodes are used.
Generally, however, it is required to set the substrate temperature at 200-300.degree. C. for obtaining an ITO layer having a low electric resistance and high transparency. When an ITO layer is formed in this manner, the ITO layer is crystallized. As a result, the etching properties are good, but cannot be said to be excellent. Further, when a polymer film or a resin substrate is used as a substrate, it is difficult to set the substrate temperature at 200-300.degree. C., and it is therefore difficult to form an ITO layer having a low electric resistance and high transparency. The ITO layer can be improved in the etching properties by preparing the ITO layer as an amorphous one (see U.S. Pat. No. 5,105,291), while the amorphous ITO layer has a problem in that the electrical conductivity decreases. Further, since an ITO layer has relatively low resistance to moist heat, an ITO electrode obtained by shaping this ITO layer into a predetermined form has a problem in that the electrical conductivity and the light transmittance is liable to decrease with time while it is used. It markedly appears particularly in an amorphous ITO film that the resistance to moist heat is low so that the electrical conductivity and the light transmittance decrease with time.
The present invention has been made to provide a novel transparent electrically conductive layer which can replace the ITO layer having the above problems, and a novel electrically conductive transparent substrate which can replace an electrically conductive transparent substrate formed of the ITO layer. It is an object of the present invention to provide a transparent electrically conductive layer which has practically sufficient electrical conductivity and light transmittance and is excellent in resistance to moist heat and etching properties, and an electrically conductive transparent substrate formed of this transparent electrically conductive layer. It is another object of the present invention to provide an electrically conductive material suitable as a material for obtaining the above transparent electrically conductive layer.
The present inventors have made diligent studies expecting that a transparent electrically conductive layer having higher chemical stability than an ITO layer can be obtained by replacing Sn in ITO with other element. As a result, the present inventors have found that an oxide which is an amorphous oxide containing indium (In) and zinc (Zn) as main cation elements and having a ratio between In and Zn in a specific range has practically sufficient electrical conductivity and light transmittance and is excellent in resistance to moist heat and etching properties, and the completion of the present invention has been arrived at.
As an electrically conductive layer formed from an oxide containing indium and zinc as main cation elements, the following films (1) and (2) are known.
(1) A transparent electrically conductive layer of indium-containing zinc oxide, formed by a sputtering method using a sintered body formed from zinc oxide containing indium in an amount of 2 at % of zinc atoms, as a target, under the condition where the substrate temperature is room temperature (see JP-A-61-205619). PA1 (2) A layer of indium oxide containing zinc oxide, obtained by forming a coating on a substrate surface by dip-coating a coating solution containing indium nitrate and zinc nitrate in an atomic ratio of indium, In/(In+Zn), of 0.80, and subjecting the coating to predetermined heat treatment (see JP-B-5-6289).
The transparent electrically conductive layer of the present invention can be easily obtained as one having excellent electrical conductivity over the above layers (1) and (2) when formed by like methods (a sputtering method and an coating and thermal decomposition method).